Quantum Levitation & Superconductors - How Does it Work?

today we're going to learn about superconductors and we're going to do it with Hands-On demos we'll talk about superconducting magnetic repulsion and also magnetic flux pinning and levitation we'll show how a superconducting levitating train can go around a circular track we'll finish up with talking about how superconductors were discovered and also the theory of how they work welcome back today one of the most exciting phenomena in science we're going to talk about superconductors and superconductivity now I actually worked in a university lab for many years actually making from scratch superconductors and synthesizing them so this is something I'm passionate about I can't wait to share this with you now a superconductor is a device or a chemical that is loses all of its electrical resistance below a certain temperature we'll talk a whole lot about the theory in just a minute now I would like to show you some of the incredible effects that you get from a device or or a material that has no electrical resistance all right so so here is the superconductor here actually this one is purchased by a company that put it inside of this plastic Puck so the actual superconductor is black and it's protected inside of there and you can let some of the liquid nitrogen in uh through the kind of through the top there so what I'd like to do is cool it down because these things only work when you cool them down below a certain temperature so this is liquid nitrogen 77 Kelvin it's about negative 195 degrees Celsius below zero so what we'll do is we'll just kind of let it get cold let me go and flip it over I believe the superconductor itself is closer to this side so we're going to leave it like this so now we have it cooled down below its critical temperature so this means it undergoes what we call a Quantum phase change where it behaves completely different than it does in at room temperature at room temperature we call it the normal state of the superconductor and below the critical temperature we call it the superconducting state so normal state superconducting state so now we're in a situation where this device has no electrical resistance so this means that it's going to try to expel all magnetic fields so magnetic fields will not be able to permeate through and so this is a little tiny ball magnet here I'm just going to drop it and we should see it bounce off the top here and you can see that the magnet is repelled by the superconductor let's do that one more time in slow motion now I'm going to get into the theory behind this a whole lot in a few minutes but the reason why it expels or repels the magnetic field is because of the electrical Eddy currents that are set up in the superconductor when you bring a magnet close to it so it's trying to generate these or induce these electric currents in the superconductor but since it's a perfect conductor the electrical currents never die down they never go away and so they expel all magnetic fields this is called the meissner effect it's it's one of the most basic effects of in all of superconductivity now there are two main kinds of superconductors there's a type 1 superconductor which were originally discovered and there's a type 2 superconductor which are the high temperature more recently discovered superconductors all of them have to be cold but the more recently discovered ones can be used with liquid nitrogen which is believe it or not a lot warmer than the type 1 superconductors needed to be almost near absolute zero to even function so even though these things repel magnetic Fields there are defects in the superconductor when you make it it's not a perfect superconductor in the sense that the lattice inside of which it's made is not a perfect regular lattice they're imperfections and because of that if you smash a magnet near and physically Force the magnet near a superconductor then it'll find one of those defects and then the magnetic flux can penetrate the superconductor but only in the defect Zone there and then what happens is the superconductor doesn't want to let that field line go it doesn't want to let it move around because again of adjacent currents that are set up to sort of lock it in place and so what you have is something called flux pinning or Quantum locking in a type 2 superconductor so even though they want to repel magnetic fields if you force it into place you can get the magnetic flux to be pinned and so the magnet can levitate and I want to show you that next all right we have our superconductor nice and chilled and what I'm going to do is drop the circular magnet on it again from a little bit higher up to force the magnetic field to go inside and let's see what actually happens and you can see in this case the magnetic field is not repelled anymore it found one of those defect zones and the flux from this tiny little magnet is now pinned inside and it really can't move if you try to push it you can get it to deflect see how I'm pushing it left and right I'm kind of pushing it back and forth It's kind of going back to the same place if you see it kind of going down to the surface it just found a defect Zone closer to the surface right if I physically bring the magnet close and force it and then just let it go you'll see It'll levitate there now again sometimes it's trying to repel the magnetic field depends on if it found one of those little defects defect zones inside here and we'll go from there so to do the levitation I'm going to push it really close Force the field lines in and then let it go and you can see it's actually spinning very slightly there as well I have some iron filings attached to the side of it that's why you can see it spinning there from a previous experiment there right so it's not magic what's really happening is the magnetic field is trapped inside the superconductor so it initially doesn't want to let any end but if any does get in through a defect zone or imperfection then the surrounding superconducting material generates Eddy currents in the material that create a magnetic field that lock that physically lock the flux lines from that magnet in place and so it can't move now if you push it hard enough it'll move but in general if I give it a little nudge in either direction it doesn't really want to move all right let me take that one out and do a little bit of a bigger magnet here let me add a little bit of cool in here all right here we go so now what I'm going to do is I'm going to drop this thing from a height and see if I can get it to lock in place camera I hope it looks really cool but but in person it's just it looks like like magic I can't even describe it with any other word other than that it looks like magic right if you push it you can get it to move but it wants to spring back into place the magnetic field lines from this magnet are locked into place and in fact I can physically move it and make it change orientation and make it lock in a different orientation so I'll push it this way for instance and then I can see it locking like that right I can grab it and remove it and then I can physically place it back and try to get the field lines to lock in place horizontally like this in a different orientation if I apply a little force a little bit lower it'll lock lower if I try to tilt it up uh it'll lock in a different orientation if I can get it to do it they're kind of finicky you get the idea as I move it around I can get it to lock in different orientations vertically now that was a circular magnet actually I think the the most impressive one is are the square magnets that I have here and the reason they're more impressive is just because you can see them spinning a little better so here I have just another magnet and this one is square let me see if I can get it to go perfectly horizontal and of course I can't all right let me see if I can get it to lock in place perfectly horizontally I'm going to push it clean and close and then let go it's pretty close to horizontal not perfect let me get the idea I can kind of push it down a little bit and hopefully get it to get as flat as I can get it that's pretty flat all right now you can see the levitation happening if I give it a nudge I can it's resisting me right it's resisting as I try to turn it but if I really work hard I can get it to spin and you can see because of the corners how how um how how how frictionless the whole process is happening you can see it kind of back and forth back and forth that's because the flux is pinning and it's kind of like overcoming the flex pinning in place there all right now what I have is a donut magnet and what I'll do I can feel the repulsion happening as I bring it down on top right but what I'll do is force the field lines into the superconductor force it to be pinned and when I feel like I've got it I can lift the whole thing up and it will be suspended underneath the magnet here so now what we have is a track made of magnets and I bet you can guess what's going to happen next if we're careful we can we should be able to levitate this thing pretty far above this track of magnets let me get it nice and cold right above the track and let go look at that you can just push it all the way around in a circle and check this out you can get it to to pin in different orientations as I move it you know I can go down closer to the track I can try to pull it up from the track and what we can do is we can pin it in place flip the track upside down and we can actually have it suspended underneath the track now if there's one thing you've noticed during this whole thing is that you can see I'm constantly adding liquid nitrogen to get it to do these have these effects all of these effects are only present below what we call the critical temperature of the superconductor now what I like to do since you've seen some of the amazing things superconductors can do for us I like to talk to you a little bit about when we first discovered superconductors what we know about them how they sort of behave and the current theories as to how superconductors work all right so now that we've done the Practical I'd want to dive into the theory and the history a little bit more Super conductivity you don't think it impacts your daily life but it actually does and so we we try to understand it so that we can build better and better higher temperature superconductors that can work in everyday circumstances so for instance when you go to the hospital and you get an MRI done scan your body the machine that you go into uses superconductors I'm going to say that again because a lot of people think this is just some lab rat doing some stuff no it's able to to we are able to build machines like MRI scanners for your well-being because there's a superconducting magnet inside of it that's carrying a very high current producing a very high magnetic field because super conductors don't have any electrical resistance we can build electromagnets out of them that generate very high magnetic fields and of course someone discovered how you can scan a body with that technology there's a whole a whole lesson there for another day on how it works but without superconductors they wouldn't function all right and then we have our particle colliders which we're learning about the nature of matter and so on and then of course the Holy Grail is we'd like to make a superconductor work at room temperature all of these superconductors have to be cooled down and so we want to make one that can uh be used at room temperature as you know we transport electricity across the world through wires but we lose energy in the electricity power dissipation because of frictions from the electrons traveling through copper wires what if we could make a superconductor wire that could travel across the world with no losses then we would reclaim a lot of energy losses due to electrical Transportation transportation of electricity so superconductors uh if we ever find one that can be cheaply made at high temperature and work at high temperature it would be one of the biggest discoveries in the history of humankind for our energy and power needs all right so let's go through the history a little bit and tell you where we're at today and then we'll close with a theory of how we sort of envision superconductors working I'm going to say in the beginning we don't have a fully baked theory for how these things work we're discovering new superconductors without really understanding the core fundamental principle behind which that they operate we have ideas I'm going to share with you them with you today but the high temperature superconductors that we have called type 2 superconductors we don't know exactly how they work there's a lot of things in science we don't know yet we're trying to find the answers so one of you guys may be the one to figure out the answers all right the first superconductor discovered was actually the element mercury it's not even a chemical compound just the Pure Element Mercury so you know Mercury is a liquid at room temperature you can you guys say you can hold it in your hand but you shouldn't because it's it's a it's a very bad for you okay it's very it's a neurological agent it's very very bad if you never hold Mercury all right but I'm just saying if you were to hold it in your hand it's liquid right if you cool Mercury down it turns out very very cold temperatures it loses electrical resistance we're going to talk about that in a lot of detail in just a second but I want to emphasize to you that when I say it loses electrical resistance I don't mean that superconductors just have low resistance right I mean it has zero resistance like literally zero there there have been experiments that have been done where you take a superconductor you run an electric current through it any wire if you try to run electric current through it it's going to get a little warm because it's of the internal collisions of the electrons and the current will be dissipated right so you lose the energy to heat and of course you have to keep the battery connected in order to circulate the current but in a superconductor you can actually connect a battery and make a superconducting loop and then you can disconnect the battery and Bridge the circuit and you can keep the electricity flowing in a loop without any battery in fact this experiment's been done many times and they can take that circulating Loop of current put it in a warehouse you got to keep it cold you got to keep liquid nitrogen on it or something even colder than that but you can pull it out uh 20 or 30 years later and this has been done and the electricity is still flowing around down in that Loop right it's not a theoretical thing it's actually still I mean they have these things in warehouses and they still check them every year and as far as we can tell there's no measurable degradation to the electricity flowing in a superconductor it literally is zero it's a quantum mechanical effect all right now we know that electrical resistance is uh has to do with the electrons colliding as they travel through the substance right and so what we can do is we can plot the resistance as a function of temperature so we can call this I'll put R for resistance really in physics you call it resistivity and that's when you take the resistance and you control for the diameter of the wire and all this stuff but for now let's just call it resistance when you shove electrons through a wire those electrons jump from atom to atom and as they jump from atom atom they collide with other electrons they collide with other uh other other electrons other other atoms that are in in the way and when they do those collisions they lose a little energy so we can use a battery to keep the electrons moving but there's constant collisions that shows up in resistance in any electric circuit we say there's resistance we try to push the current we can overcome the resistance with a battery or some other voltage source but there's an inherent resistance into pushing electrons through a wire because of collisions remember even at room temperature we're nice and comfortable but atoms are jiggling around violently at room temperature because you know absolute zero we're very far away from absolute zero you know we're almost 300 degrees Kelvin or you don't really say the word degree 300 Kelvin right above absolute zero I'm rounding a little bit here but you get the idea we're very far from absolute zero so as you get colder and colder the thermal motion of everything starts to slow down now you can never get to absolute zero but theoretically as you approach it you start to slow down more and more and more so as we measure the resistance of something right over here at a high temperature let's say this is like room temperature is about 300 Kelvin or so 293 Kelvin right as we cool down I'm going from the left the right to the left because these are high temperatures like you could say this is 300 Kelvin right and as we cool it down to like 200 Kelvin 100 Kelvin here's like zero Kelvin over here we expect the resistance to go down right we expect it to go down but uh in the early 1900s these experiments were being done and we really didn't know what the resistance would do when you got really really really cold right there's a couple of you know kind of kind of things you might wonder about would the resistance just keep on going down and eventually it would at zero Kelvin or close to zero Kelvin we would have some minimal resistance would the resistance kind of get and start to bend over more and maybe land at uh you know maybe not zero resistance but some lower value of resistance kind of as we get to absolutely zero close to absolute zero or with some other effect kick in and somehow somehow the resistance goes up as we get colder and colder and colder most people thought that the resistance would just go down down down down down and as you get to ultra cold temperatures you just have some very low resistance right but actually what happened is the following when you cool a superconductor in certain materials Mercury for instance call this Mercury this is the symbol on the periodic table for mercury as you get down colder and colder and colder the resistance gets lower and lower and lower and eventually it departs and then it just goes straight down right and it goes goes all the way down so that the resistance is actually zero at a temperature above zero Kelvin this is not okay this is zero Kelvin right so for mercury this temperature is about 4.2 Kelvin this is really really really cold Mercury was the first superconductor discovered and it has what we call a critical temperature at 4.2 Kelvin now I want to stop for a second and I want to to transition our thoughts away from superconductors let's talk about water we know all about phase transitions there are certain temperatures that water behaves differently if you get it down to zero Celsius it changes from this liquid into a solid hard substance we call it ice and if you melt it and it goes to water then it can flow if you take the water and you heat it up and heat it up and heat it up to about 100 Celsius then it changes phase again from liquid to uh to to vapor and it can float away and has totally different properties so ice and liquid water and gaseous vapor water have completely different characteristics you know the ice is solid and hard the liquid flows easily and the vapor can expand to fill the room completely different situations we have connect did all those together through the theory of matter and we know why those face changes happen as we talk through the superconductors I really want you to start to think about the difference in the way the material behaves as a phase transition I'm going to call it a Quantum phase transition that's what people typically refer to but just keep in mind that All Phase transitions of any matter is quantum because all matter is made of things that are quantum things these molecules they're all Quantum objects right but the behavior of superconductors wasn't noticed because it was impossible or we didn't have the technology to make temperatures that cold or to cool things down that far until the year 1911 was when it was discovered so it turns out if you take Mercury and you make it very very cold eventually it gets hard and it's to a solid it turns into solid Mercury and then if you keep making it colder and colder and colder and colder and colder the resistance goes down down down down down but eventually at around 4.2 Kelvin the resistance disappears completely all right zero resistance now Mercury is what we call a type 1 superconductor I'm gonna different differentiate that with a type 2 super after a little later but I need to get a little farther in the lesson essentially there are two broad classes the type ones really only work at very low temperatures this is just a few degrees above absolute zero very very cold and you need liquid helium to actually even do the experiment liquid helium is very expensive it's very expensive to make very very cold to to to take the heat out of something enough to make liquid helium so it's a very big deal if we can make a superconductor make work at a higher and higher critical temperature all right so I'm going to go through a couple of discoveries so people did lots of discoveries I'm skipping over a lot there are actually quite a few elements on the periodic table just by themselves they superconduct at very low temperatures six Kelvin eight Kelvin 10 Kelvin a lot of experiments were done trying to find materials that work at a higher and higher temperature finally a breakthrough happened and I'm skipping a lot of history but a breakthrough happened in 1986. so in 1986 a compound was discovered uh they call it uh l b c o this is lanthanum uh barium copper oxide this is not carbon okay and this is not the actual chemical formula the subscripts I'm not writing the subscripts here but you it's called lbco lanthanum barium uh copper oxide you're going to find out that the copper oxide family those are the superconductors I worked on for three or four years copper oxide family is uh extremely prolific with lots of different superconductors coming out of that family the first one was this and uh this one here and the critical temperature here the critical temperature here was 35 Kelvin all right so not that cold but definitely warmer but you still need a a liquid helium to cool it to this temperature right because liquid nitrogen which is what I was using in the demo is at 77 Kelvin I'm going to write that down I'm going to write that down I guess over here so ln2 this is liquid nitrogen is about 77 Kelvin liquid nitrogen is actually really cheap right it's still way colder than room temperature if you think in Celsius that's about negative 195 or so Celsius so that's very very cold but it's very cheap to make liquid nitrogen compared to liquid helium so this you still need liquid helium to uh you know to operate or to to to bring it down below its critical temperature and see the superconducting effects right but also in the year 1986 1986 1987 there was a lot of stuff going on they replaced the lanthanum element here with yitrium on the periodic table yitrium barium copper oxide my professor actually was involved in this heavily in 1986 and you know Dr Paul Chu is fairly famous for for this for this high temperature superconductor Renaissance the temperature of this guy was around 90 Kelvin the critical temperature that means that since liquid nitrogen is at 77 Kelvin but this uh yitrium barium copper oxide Works above 77 Kelvin that means that we can cool it down with liquid nitrogen and bring it below its critical temperature very very cheaply because the liquid nitrogen is cheap I know that it looks so weird with the smoke and all that but it's much cheaper than liquid helium I've done experiments with liquid helium and you need a much more sophisticated setup because it's just so hard to keep helium cold at that temperature you need a super super Industrial vacuum jacket very difficult to do it gigantic machines just to keep a tiny amount of helium cold enough to do anything with it whereas liquid nitrogen I can put it in a thermos and it'll be okay for an hour or so before it boils away all right so that was a big deal that was before my time I was not in college in 1986 but that's when the the Renaissance for what we call high temperature superconductors when you see someone refer to high temperature superconductors it generally means a superconductor that works with liquid nitrogen anything above 77 Kelvin right now in 1993 which is actually when I started University there was a family of compounds that was discovered and these are what I worked on so in 1993 uh there was a family of com it's a whole family actually of compounds and uh this one is the following it's Mercury barium calcium copper oxide and there's a different number of oxygens depending on how you make it and also uh so so we called this Mercury one two two three that's what we called it but actually there's a whole family you can have mercury one two one two so you can have a one for the calcium and a one for the copper that's also superconducting the temperature of this guy is you ready for it a whopping 138 Kelvin right much much higher than this one which was you know almost 10 years earlier right and I'm going to mention it a whole lot more but these copper oxide superconductors the interesting thing about them is if you apply pressure to them while they are cold like literally if you take the puck put it in a vise and squeeze it while it's cold the application of pressure actually increases the critical temperature physically forcing the atoms in the lattice closer together actually increases the critical temperature and we're going to find out in just a second all the way up to very close to room temperatures we actually uh have already discovered I'll show you in just a minute as we write it down all right so a Mercury barium calcium copper oxide right now before I go on since I spent a few years of my life making these things I'll briefly explain how they're made so Mercury oxide is a powder these are all going to be oxides you buy them from the chemical company Mercury oxide barium oxide calcium oxide copper oxide you don't need any Oxygen because the ox the oxygen is already in the other compound so four compounds they're all powders and what you do is you mix them together in the proper proportions you grind them together to get maximum contact of the different atoms and then you press them into a pellet literally into a puck you do that in a vise and you apply pressure and you put it in a puck you take that Puck and you put it inside of a glass or a quartz tube which is sealed and you put that inside of an oven or a furnace and you bake it so it's like an Easy Bake Oven you put it in there at about a thousand degrees a thousand degrees for a five or six hours it depends if you try six hours ten hours 12 hours that's all the experiments we were doing back in the 90s you know how much do you put in there what temperature how long all that stuff out after six or seven hours the puck that you have which was Gray is now black and when you measure its resistance at room temperature it's not a good conductor at all but if you cool it down it becomes a superconductor Okay and like I said this is a whole family of superconductors here as well all right now I moved on from superconductivity and so lots of research has happened between now and then I'm not really up on all the latest but I'll tell you the highlights in 2019 I was certainly not involved in any of this stuff a compound called lanthanum hydride was at a very high critical temperature so I'll write it down I guess I'll write it down here so in 2019 lanthanum hydride so lanthanum on the periodic table and 10 hydrogen atoms has a critical temperature of 250 Kelvin look at this room temperature is just under 300 Kelvin so 250 Kelvin is really close to room temperature but there is a catch the only way they were able to get 250 Kelvin is by applying pressure to this thing and not just a little pressure 170 Giga pascals now you might say uh who cares what's a pascal well I'll tell you I'll just go up here and I'll tell you that one atmosphere so what you feel right now is is somewhere on the order of about a hundred kilo pascals so what you feel right now uh on your skin from atmospheric pressure is about 100 kilopascals so this is in kilopascals this is giga billions of pascals so this is not just a little bit of pressure this is an enormous amount of pressure and the way you apply this pressure actually you don't just use a vise you have to have a diamond diamond is the only material that can withstand the pressure you have a diamond Anvil with sharpened points and you put the superconductor between it and you apply this incredible amount of pressure while you cool it down and it turns out that they can get a superconducting critical temperature up to 250 Kelvin right and then in 2020 which was at the time of this recording literally just two and a half three years ago uh I don't actually have the entire formula here but it was it was a compound with hydrogen carbon so not calcium carbon and sulfur one of these days I'll look it up and see what it actually is has a critical temperature on the order of 288 Kelvin at uh 270 Giga pascals now 270 gigapascals I can't even fathom it okay this is like this is like the center of Jupiter kind of kind of pressures all right so the point is you might say well this isn't practical who cares if they made a room temperature superconductor at so many Giga pascals this is the way science often is see when this super conductivity was first discovered scientists really didn't think it could work at all above 20 or 30 Kelvin just theoretically it just it just wouldn't because we didn't understand how it worked and their best theories basically just thought that the process wouldn't even occur above a certain temperature I mean after all ice and steam you know they have very fixed phase transitions right so maybe this only works at certain temperatures and that's it maybe getting room temperature superconductors impossible well what this proves is that we have different compounds that operate very close to room temperature but we have to apply a lot of pressure even though we have to apply a lot of pressure and so it's not practical to build a wire out of it right now what it tells you is that theoretically it's possible because not only is it Theory it's it's it's demonstrated that superconductivity can exist at a high at a high temperature at a room temperature even though the pressure has to be high it gives you an existence proof it shows you hey this does happen just like the first superconductors happened at 4.2 Kelvin now we have them operating way higher than that this tells you that there is such a thing called the superconductor and this tells you that it's possible to get that critical temperature up close to room temperature all we have to do is understand the theory enough to know why the pressure influences it in the way that it does then maybe we can engineer a compound that works without the pressure so it's one thing at a time right first you just you discover penicillin as an antibiotic and you're like wow antibiotics exist and then you go research and have we now have whole families of antibiotics so it's my firm belief that hopefully in my lifetime but definitely in in yours that we will have superconductors that are room temperature and that that function at room temperature all right so let's go in in a little more detail and talk about how this works a little bit more so we have this concept of zero resistance and I already drew a graph on the board but I want to draw another one because I want to show you how we measure these things so this is the temperature in Kelvin and this is the resistivity or the resistance right and as I said here you have room temperature is actually uh it's 300 Kelvin but it's actually 293 Kelvin I was just rounding there so this is room temp this is a nice comfortable day the resistance of most substances is up here and as you cool it down then it just drops right off a cliff and goes down at some critical temperature which we call TC what we want to do is move this TC way up here so the critical temperature is like somewhere near room temperature that's what we want so it's always super conducting even without any coolant at all that's what we want now just because I've done it a few times I want to share with you how do we measure this how do we actually measure the critical temperature what you do is you take your superconductor which is because I'm drawing a rectangle but it could be a puck in a circle and what you do is you make four contacts on the surface of the superconductors so there's one contact there's another one here's another one right here and here is another one right here and in order to measure the resistance what you have to do is you have to run an electric current through it and you have to measure the voltage that's occurring across it so what you do is you hook the outer ones to a current source so I'm going to put I right here and it's I guess I'll put little dots right here so it's connected right there and so the current kind of goes around through here through the superconductor and then back back around so that's there's this electric current uh circulating there and then what you do is at the same time you're measuring this electric current or you're sending this electric current you connect a voltmeter across here so I'm going to put volt but this is a meter here so this is sending electric current through and this is measuring the voltage across the innermost pads now why do we have different pads here well if I take a regular voltmeter like from the hardware store and just connect it what the voltmeter is doing is it's sending current through and it's measuring the voltage from the same probe tips the problem is if you touch a superconductor you know with the same probe tips if you don't separate them like this what's going to happen is there's a contact resistance from where the probe tip touches the superconductor so if you measure the voltage at the same place the probe tips are touching you're going to measure the contact resistance since the superconductor gets all the way to zero then you don't want to measure any any other resistance anywhere other than the superconductor so you have to separate uh you have to separate the current going in and the measurement of the voltage now we know from Ohm's law from Electronics V is equal to IR right the voltage across anything is equal to the current flowing times the resistance and so you can calculate the resistance as V over I so what you do is you send current through here and so you know what the current is and you measure the voltage across this little interior piece right here and as the voltage approaches zero or goes to zero right as the voltage goes to zero then the resistance will go to zero also because the numerator will go to zero and so you plot this the whole time you're basically sending current through measuring the voltage and as you slowly cool it down the voltage is going to go down down down down down which means the resistance is going down down down down down down eventually after it's superconducting even though you're sending a current through there's no voltage drop across it if you've done anything with circuits you know that if you have a perfect conductor which you never can have outside of a superconductor then there's no voltage drop but across a superconductor there really is no voltage drop because you don't have to supply any voltage to continue the current flowing it's just going to flow for free for lack of a better word all right and one last thing I'll say is that in order to connect these things because I had to do it for so many years you get under a microscope and you get this NVM metal it's a metal on the periodic table called indium it's quite expensive but it it spreads like peanut butter it's kind of a soft metal at room temperature and literally under a microscope with tweezers you spread the indium metal on the superconductor and you connect your wire you put a platinum wire here and a platinum wire here and a platinum wire here and a platinum wire here and you spread this Indian metal and you spread it on top of this Platinum wire so at the end of the day under the microscope you're looking at four contact pads four wires coming out and you connect them to a measurement device and then you slowly cool it down by slowly lowering it in liquid nitrogen and every time you drop it down closer and closer to the liquid nitrogen you measure this resistance and then you get a graph that looks just like this maybe one day I'll do that it would require a lot of work so I'm not sure I have I have this set up here to do it it would be fun to do it but that's how it is done all right zero resistance literally exactly zero resistance why do all of these levitation effects happen okay now this is when it gets into Theory and I can describe to you what we what the current theories are but just know that it's all quantum mechanical effect and we don't know exactly how high these type 2 superconductors work remember type one are the superconductors that only work at very low temps type two are all of the numerous superconductors that work at the high temps right so we know from electricity and magnetism that magnetism and electricity kind of go together like peanut butter and jelly uh I'll say that again magnetism and electricity go together like peanut butter and jelly they're really manifestations of sort of the same thing which we call the electromagnetic field so because of that magnetism can induce or produce electricity and electricity flowing electrons also produce their own magnetic field right so how how do we use that we use it every day in a motor when you have a motor and you run an electric current through a motor and it spins what's happening is you're running an electric current there's a magnet in there and so there's a force that's exerted a magnetic force and so the thing turns if you run it backwards and you get water or steam to turn a turbine or a generator then you have the same setup you have magnets and you have wire and you're turning the coil of wire in the magnetic field and you're generating electricity so what happens if you have a perfect conductor and I mean a literal perfect conductor if this is your super super conducting puck right here and it's cooled down below its critical temperature and then right above it I bring a magnet here so it's north and south pole of a magnet and I begin to lower it down well we you know draw these you know the field lines around you know the magnet like this and by the way these field lines you know there's a lot of debate on what you want to believe right are they real are the real field lines there well I'm not really sure there are good calculational tool they're a good model of the universe right they're a good model of the universe to explain how things interact are there really invisible lines I mean no not really maybe depends on how you want to think about how real things really are if it exists in a way that helps us predict what's going to happen next if it gives us a self-contained way of predicting things and they might as well be real but are there really invisible Lines no not really it's just that these things behave in a certain way that are nicely predicted if we envision an invisible field with the with this geometry around it okay so that's a little bit into philosophy because what is real what is not real but magnetic fields electric Fields they're they're they exist right on the edge of like uh uh if you believe they're real or not just depends on on on your view of reality and if you think that something is real if it has predictive power or not all right so that's my little Spiel there but anyway here here's a genetic field if you drop this thing down into the superconductor where the field lines attempt to penetrate the superconductor right what's going to happen it's a perfect conductor we know from Maxwell's equations that govern electricity and magnetism that anytime you have a changing magnetic field in a loop of wire it generates an electric current but this is not just a loop of wire it's a solid conductor a perfect wire perfect so you can think inside of here is like there's a bunch of current Loops which are all like infinitely close together like it makes a whole gigantic Puck as soon as you bring this thing close like when I was dropping the magnets the magnetic field starts to try to penetrate this conductor but as soon as the magnetic the first magnetic field line attempts to make contact with this conductor immediately at that moment a electric this magnetic field induces an electric current to flow in the connector we call it an eddy current Eddie current is a nice little term uh that just means that these little uh these little currents are existing as little miniature circles somewhere in the conductor I don't actually know if it's oriented this way I haven't done the right hand rule to figure it out but as you bring this magnetic field down some current is going to be generated somewhere in this Puck and that current in a superconductor will always be generated in such a way that this current produces its own magnetic field remember electric currents also produce their own magnetic field but anyway the magnetic field from these Eddy currents will be produced in such a way is to try to cancel the magnetic field approaching and it has to do that by the way because if it didn't then you have a larger and larger magnetic field because it's superconductor and it's perfect you would have a magnetic field that would grow forever and with infinite because magnetic fields store energy you would have a free energy machine and that can't happen so in the real world when you bring this thing down the magnetic field begins to cut through this conductor it generates an electric current which produces its own magnetic field which tends to cancel this magnetic field approaching so that the end result as we say superconductors do not allow any magnetic magnetic fields inside of them but it's not because there's a person standing at the door slapping away the magnetic field it's not because they care it's just because the Eddy currents that are generated in the surface of the puck at the the first minute presence of magnetic field immediately generates a counter magnetic field that cancels the one approaching so no magnetic fields can exist inside of a superconductor so you might say what basically that looks like is as soon as the field approaches instead of going through like if this were just you know plastic or something the magnetic field would just go right through but in a superconductor the magnetic fields literally go around and I'm not drawing any arrows of course magnetic fields have arrows but you get the idea all right now I meant I mentioned many times as we did the experiments I told you I've bounced it off and I showed you how it tends to to repel magnets but then I showed you if you kind of overcome the repulsion uh the that's called the meissner effect when it tends to expel magnetic fields due to these Eddy currents that are generated if you kind of physically push it in there and and overcome the that resistance then for type two superconductors only type 2 superconductors you can force the magnetic flux to penetrate the superconductor and then once it's inside it's actually Frozen in place it's locked in place so I like to talk a little bit about that uh we don't have all the theory here all right so I want you to know that what I'm telling you are sort of like the current theories but it's definitely not you know not not written in stone because these theories are the superconductors are the Cutting Edge of science so we don't actually know exactly how they work but the theory is that let's see if I'm gonna have a room on this board yeah in a type 1 superconductor this is how it would behave so type one type one superconductor they were it would allow absolutely no magnetic field inside of the material and it would be purely repulsive effect no flex pinning right but in a type 2 superconductor which is the Mercury barium calcium copper oxide or the yitrium barium copper oxide those and all the other newer ones those are all type 2 superconductors so there's basically a lattice of the bariums the atriums and all the things that is made of the superconductor in a regular lattice right but that lattice is not perfect in other words if the material is mercury barium calcium copper oxide in these proportions with the subscripts here then maybe once it once I pressed it in there and I baked it then the lattice may be at this location right at this point isn't perfect maybe instead of two bariums at this spot it's supposed to have it only has one or it has three of them or instead of you know enough oxygens over here it has one less oxygen in some sort throughout the lattice of the structure it's not exactly perfect in all locations so the local superconducting effect in certain little zones in these type 2 superconductors is it really perfect throughout the whole thing and the idea is we when we force the magnet in there if it finds one of these local defects then the flux and the magnetic field can penetrate locally in that region we call that a flux tube believe it or not right so in that situation what you might have a little cartoon for it anyway would be the following right so I'll say type 2 type 2. so what you have is you have the magnetic field for the most part being expelled you know from uh from the superconductor but then if there's a defect right here then maybe what you have is let me get to my picture let me make sure I draw it the way I want to draw it then maybe right here the flux can kind of penetrate right in this one little defect Zone something like this right and right here and maybe there's another one right here where the flux can kind of kind of like be concentrated and just make it through this one little Zone which is not really super conducting in other words the superconductor as a as a uh as a bulk object the whole thing we say is superconducting but really if there's imperfections in the lattice of the structure of this of this chemical compound and it's not perfectly Mercury one two two three at this location then maybe right at this location it's not exactly superconducting so at that one location a tiny bit of magnetic flux can make it all the way through to the other side and any flux coming in instead of going around will be able to locally penetrate right here but the thing is is that once it's locally penetrating through to the other side right there then if you try to move the super the magnet that's generating this left or right notice how it was kind of like staying in place we call it flux pinning right as you try to move it away from that flux tube from that imperfection as soon as you move it then Eddy currents are generated nearby which tend to push it back to the imperfection that's why it's pinned in place that's why the levitation happens we've all played with magnets everybody has you love to to repel magnets it's just magical but notice that Matt it's very unstable magnetic fields can repel each other but they're very unstable that's because in two magnets nothing is pinned the magnets are repelling each other because of the interactions of the magnetic field but it's almost like balancing a pencil on your finger it's very unstable in this situation it's not so much that it's being repelled it's that it's being trapped the flux is being pinned in place and that is why when I lift up it tends to resist me lifting up when I push one side I can get it to reorient itself because the flux can be pinned in whatever location I release to through these specific sites which are defects in the bulk superconductor that is a theory that may be proven wrong but that is the current theory as of right now okay so type 1 superconductor accepts absolutely no flux it's just a repulse effect effects type 2 superconductors operate at higher temperatures they're more complex they have a crystal lattice and they can pin the flux in magnetic levitation is what we have possible with high temperature superconductors so if you want to build a train or you want to levitate a car you would do it with a type 2 high temperature superconductor all right now before I forget we've mentioned that superconductors have a critical temperature right and this critical temperature right we call it TC critical temperature well actually there's some other things that you can do to superconductors to make them lose their superconductivity right that are different than just the temperature it turns out that if you uh if you have a superconductor and it's in the superconducting state and you put too much electrical current to it through it then the superconductivity will stop and that's called the critical current density so I'm just going to say JC J is a is the symbol in physics for current density this is the critical current density so I'll say current density that's JC so if you have if you have a superconductor and you put a hundred thousand amps through it it's going to immediately stop superconducting somehow by putting too many electrons through the bulk material it disrupts whatever is causing the superconductivity in the first place and the thing will immediately transition back to the normal State and start to heat up even though it's very cold right secondly or I guess I should say thirdly I wish I wasn't running out of space there is something else called a critical magnetic field we call it BC so this is the magnetic field right uh right now I'm putting these magnets levitating them on top of a superconductor everybody's happy but if I take a very very strong magnet very strong magnetic field and I immerse the superconductor in a super super strong magnetic field above some critical magnetic field strength superconductivity turns off so there's not just one thing that can turn off the superconductor there's actually three things there's probably other things too but the three main things if you make the temperature too high the superconductivity stops if you make the electrical current too high the superconductivity stops if you immerse it in a magnetic field that's too high the superconductivity stops all right now I'd like to close with the current theory as to how this actually happens but I want to caution you a couple things first I want to caution you that this is just a theory right in years to come someone will come up with a better Theory and that theory will model the situation better and it will yield hopefully more discoveries but this is the theory that we have right now it was put forth a long time ago secondly this Theory only really applies to type 1 superconductors it isn't really a good theory for type 2 superconductors because the flux pinning and the behavior of type 2 is actually I'm leaving out a lot it's significantly different than type one superconductivity the superconductors but the thing is we really want to make more type 2 superconductors that work at higher temps so people are searching for a theory that explains it and when I say Theory the these theories are not just put forth like some people just throw in darts at a board they have predictive power so we don't just put forth theories and say oh it sounds great we put it forth and we asked does it predict anything so the theory that I'm about to show you and share with you is called BCS Theory and it does have some predictive power and it has predicted some superconductive superconductors which were then discovered okay BCS stands for the names of the authors that put forth the theory uh there okay let me give you the big picture and then I'm gonna have to dive in a little deeper resistance in a wire happens because when electrons flow through the wire they're basically in undergoing collisions the whole way through right but somehow as we cool the temperature down of lots of different materials then the resistance completely goes away so somehow the interactions of the electrons with the other atoms and electrons that are in the in the material it somehow disappears it it's more analogous to a phase change like water to ice or ice to get or liquid water to gas or vapor a phase change this is called a Quantum phase change where the the bulk properties of what an electron is inside the material behave differently I want you to remember as I talk to you that you don't know what an electron is neither do I neither does anybody we have theories we in in grade school we talk about electrons being little balls that Collide right they're not balls in quantum mechanics we describe the electron as a wave function it's like a traveling wave I know it sounds weird but that's what the current theories are and believe me they must have wave-like properties because electron microscopes which we use every day kind of rely on electrons behaving as waves so we know they have a wave character right and waves can interfere and and waves can add and subtract and waves can do all kinds of things that waves can do but we also know that electrons have a particle like property they Collide they bounce off of things and waves don't seem to do that so electrons whatever they are have characteristics of waves and particles just like photons of light also have characteristics of waves and particles right but I'll give you the big picture the big picture is that light it's called a photon it's a particle of light that has a wave-like character and light is able the photons of light are able to if I had a bucket of photons then they would all be able to coexist in the same Quantum state in other words they would be able to they would be able to have the same energy level the photons could all be literally sitting on top of each other photons don't really Collide bounce off of each other you know that take a flashlight take another flashlight do you see anything bouncing do you see any photons bouncing off of photons no they bounce off of matter they bounce off of air they bounce off of mirrors they bounce off of planets but photons don't bounce off of photons they can exist in the same place with the same color with the same energy no problem that's how we make lasers laser is a very pure color light and the reflection back and forth in the laser cavity makes a bunch of photons in the same Quantum state right now electrons are different electrons seem to bounce off of each other they can Collide and so they can't exist a bucket of electrons cannot exist in the same Quantum state they're fundamentally different electrons cannot all be together at the same energy in the same place occupying the same space if you try to take an electron and put it here and you put another electron right on top even if they weren't charged because I know you're thinking oh they're repelling forget about that they still cannot be in the same place because they are fundamentally different than photons right but I'll give you a hint the punch line and as we talk through it I want you to remember this this is important in the superconducting state the electrons that are flowing start to behave like photons I'm going to say that again because that's really important the electrons that are traveling through the superconductor as a group as a conglomerate they start to behave kind of like photons they undergo a phase transition a Quantum phase transition where they stop acting like these individual balls that bounce off and and can't exist in the same place and they can't be in the same energy the electrons of every other atom we examine and they start to transition to a different kind of entity where they can exist in the same place and they can act as a group and they can all be in the same energy in this state the electrons can move through the material without interacting with any other adjacent electrons because they're all remember the atoms are almost all electrons right the protons in the neutrons in the middle are there but the atom is mostly completely empty space surrounded by a sea of electrons so as an electron goes through it's not going to interact with a nucleus it's going to interact with other electrons mostly but if all the bulk electrons start behaving as one whole entity thing then they're not interacting with each other they're not bouncing off of each other they're not colliding with each other now if you start to warm this thing up and make it agitated and disrupt this Quantum phase change then suddenly they start to behave like regular electrons again and they start colliding and resistance starts to happen that in words is what is happening but I want to draw some pictures so that we can go a little deeper together and I find this fascinating I hope this is you know this is uh uh will be expanded upon to Encompass the type 2 superconductors but we'll have to see in the future all of matter and all of energy everything you've ever seen in your life is really governed by one of two classes we call them bosons and fermions I promise it's got weird names but these will be easy to understand so you have something called a boson these are all named after people right and you have fermion these are classes these are not particles these are classes so the example that I want you to think of for a boson is called a photon whoops I can spell Photon right I mean it kind of sounds the same boson Photon they sound the same right fermion the example I want you to think here is an electron fermion electron I mean it doesn't really help as much but you get the idea boson Photon they sound the same fermion electron now there are other kinds of bosons there are other kinds of fermions but I don't want to talk about those because we'll spend all day let's just talk about photons let's just talk about electrons okay now here's the part where I have to start to get a little abstract a photon has a characteristic it has energy and position and all that stuff but it also has something called spin I'm going to talk about it in a second and the spin is an integer that's what makes it a boson anything that has a spin that's a whole number like this is called a boson right now anything that has a spin that's not a whole number like an electron has a spin of one half anything with a fermion which has some kind of fraction doesn't have to be a half but it has to be a fraction it's called a fermion right now I want to come back to what spin is in just a second please I know you're thinking what what is that are they really spinning just just hold it because I I promise I have all the same questions you do so just kind of let me get through right uh we will come back I I promise you I remember in ninth or tenth grade I spent like weeks trying to understand what spin was and then I realized nobody really knows what spin really is so join the club if you don't understand what spin is nobody does not even the experts know what spin really is all right what this means is that in terms of photons all photons can exist in same state Quantum state what I mean by Quantum State they can have the same color they can have the same intensity they can have the same energy and they can have the same position right this is what happens inside of a laser we use this all the time and this it means if you draw like an energy level diagram where this is energy then what it means is that some minimum energy right here then what you can have is you can have a bunch of photons just hanging out at this low energy state this is energy on this axis some low energy so whatever the color of the light is red light has a certain energy they can all be all the photons can be inside the laser cavity with exactly the same wavelength of light and it can all just be passing through each other they don't interact with each other at all they can exist in the same place this is what photons are right we don't know why spin of one uh spin of one makes it do that but it does sorry if you want deeper explanations I'm not going to be able to help you because nobody understands exactly why quantum mechanics behaves the way it does but we do know that all of a matter is either a boson or a fermion spin one means that they can all exist in the same state like this same location same place same quantum number same everything this is how we make lasers of exactly the same color coherence all the stuff that makes a laser what a laser is now because I've led up to it hopefully you should know the fermion doesn't behave in the same way can't can't uh share same Quantum state you may remember from chemistry something called the uh the poly Exclusion Principle when you learn about atoms you learn okay here's a nucleus of an atom let me put an electron there okay Boop now there's an electron you can think of it going around but it's not really orbiting but let's just think of it like going around for now now you take a second electron you can put it in there has to have a different Quantum State the quantum numbers have to be different for electrons to exist around an atom you put a third electron it it can't be on top of the two that are there it doesn't behave like photons it has to be somewhere farther away with different quantum numbers than the ones that are lower and if you put a fourth and a fifth and a sixth electron in place they have to be different Quantum states that means the electrons have to get farther away with different quantum numbers and that's why on the periodic table when you look at iron with a lot of electrons then the electrons are all the outer ones are very far away the inner ones are very very close because electrons cannot be on top of each other with the same Quantum States photons can and the difference between them is basically what we call spin I'm going to come back to it in just a second before I do that let me draw a little picture of this so in the nucleus this is protons and elect and neutrons in the nucleus right here's an electron right and you learn in chemistry right that you can in the same lowest orbital you can have two electrons in the same lowest orbital but in order for them to exist in the same lowest orbital this one has to have uh like an up spin of one half and this one has to have a down spin of one half so when I told you that this thing has spin there's one little Nuance yes spin is a number but spin also has a direction I'm going to talk more about Span in just a second but up spin of one half is different than down spend of one-half so two electrons can exist in the same orbital as long as one has up Spin and one has a downspan of a one-half but you can't put a third electron in that same orbit with those because electrons can only have an up or a down of one half and so if you put another one here it can't have a different Quantum state so it cannot exist in that lowest orbital there we call it the 1s orbital energy level one s is the shape of the orbital you'll learn about all this stuff in chemistry and quantum physics and things like this right if you put a third electron in place it just can't be here it has to be farther away in a farther distance from the nucleus again different quantum numbers as you start piling more electrons in place it's because of the difference between electrons and photons because of the class of the particles there now what is spin I'm going to give you the punch line nobody knows nobody knows all right but what we do know is that electrons have what we call angular momentum you learn in physics about something called angular momentum if you take anything and spin it in a circle like a merry-go-round a child just fling something around on a string anything rotating has an angular momentum and we know that electrons have angular momentum right we know this why because moving charges generate magnetic fields if I take a coil of wire literally you can do this experiment if you run an electric current through a coil of wire you ever build an electromagnet and you put the nail and you wind the wire through it the electrons are going through that wire then they generate a magnetic field and they magnetize they all add up and they magnetize that nail right that's an electromagnet we can build these things we know they happen but we know that if you take a single electron just one electron I'll just put a right here this thing is called an electron it's got a negative charge just one electron there's no atom there's no wire there's no nothing just one electron in an experiment and you very carefully measure that electron in addition to having a negative charge it also has a very small magnetic field let me say that again the electron we learn in grade school they have positive they have negative charges right and so like charges repel electrons repel but also what you don't really learn in grade school is that electrons have what we call a magnetic moment they have their own little magnetic field and in terms of our framework of physics the only way it can really have a magnetic field is if it were a tiny little coil of wire rotating because we know that coils of wire generate a magnetic field so if you take a single electron and you could rotate it or if you take a single electron and just make it go in a circle it should make a magnetic field if you make the circle smaller and smaller and smaller so it's not really making a circle anymore it's just a single kind of like entity rotating then it would have angular momentum and it would also have a very small magnetic field and electrons have a small magnetic field we know this to be true and the magnetic field can be oriented in one of two directions corresponding to what we call spin we say that the electron behaves as if it is spinning now we know it's not spinning right we know it's not a ball we know that all of these things photons electrons quarks all of it they they're Quantum wave functions so they're not little balls that spin but whatever they are they have a wave like character and they also have some sort of magnet uh some sort of quality to us that to us looks like angular momentum it looks like a spin you can think of it as a spinning ball so instead of uh when you see spin I want you to not think oh this thing is spinning I want you to think it has angular momentum and that means it has a magnetic field we don't know if it's spinning or if the wave function is different than how we have it modeled or exactly where the spin comes from but it behaves as if it were a tiny tiny tiny part of local spinning and the electron has a magnitude of a spin that has to be some fraction and that fraction is one half all fermions have spins that are one that are fractions and the electron is a type of fermion that has a spin of one half now the photon has a spin of a whole number one and why you say y one half y one you're starting to ask questions Nobody Knows the answers to I shouldn't say nobody but very famous people Richard Feynman said I think I can safely say that nobody understands quantum mechanics if if Richard Feynman doesn't understand it nobody understands it this is because when you start asking deeper questions you know I could keep going well why does it spin I don't know nobody knows why you at some point you have to get to a point where you I don't want to say stop asking why because I always want us to ask but you have to accept that some things are just the way they are and we have no capability currently of probing into some of those questions until we discover maybe other aspects of the universe maybe they're spinning because they're all connected to another Universe somehow maybe all the universes are connected together and the thing things that we see as spanner is like their connection to another I'm just making this up I don't believe that I'm just saying it could be related to something that we have yet to even discover that's why it doesn't make a lot of sense to us but I need you to know that a photon has a spin it looks like a particle that is rotating with a span of one and a a whole number whole number one and a fermion has a spin of one half and because the spins are different elect photons can all exist in the same Quantum State and electrons cannot that's very important all right so now we have this information we're able to finally understand the BCS theory of how superconductors work all right first let's talk about a normal conductor normal conductor so what we'll do is we'll draw a wire right and this is a cartoon right it's obviously a lot more complicated than this but inside of this wire our nuclei which I'm calling positives right now the nucleus really of an atom has protons and neutrons and so I'm considering the entire nucleus to be in these little balls so uh the the charge of the nucleus doesn't matter I'm just putting them there and just so you know they're protons and neutrons in every one of these positive spaces in a wire like a copper wire there are electrons bound but the outermost electrons are very weakly bound to the atom they're very easy to get them to move that's why they're good conductors like an insulator like plastic or something the electrons are very tightly bound and so they can't move and jump between atoms very easily so they don't conduct electricity very well but in this case uh they do so if um if I had an electron let's say right here this is an electron and I put a voltage across this wire to try to push this electron right then this electron is going to go here oh it's going to crash into this thing and maybe there's an electron here as well and then it's going to crash down here there's a crash up there into something I'm not drawing all the other electrons that are here they're electrons everywhere in here it's a sea of electrons and and these and these positive charges and also these negative electrons they're jiggling everywhere at room temperature everything's violently moving everywhere room temperature and so these things are scattered everywhere off of mostly off of other electrons that are in this but as a group they even though they're bouncing off of each other as a group they're still kind of making their way over this direction and that's what we call electric current flow but there's a lot of losses because every time it collides with another electron or something else in the way it loses a little energy now we have a battery connected which is always putting energy in and fresh electrons and so the process can continue but we're losing energy we see that in an electric circuit as heat the wire will begin to heat up the toaster if you think of your toaster oven you put the electricity is going through it it gets very very red hot or a light bulb getting it red hot that's uh what we call ohmic losses power loss due to collisions due to frictional heating there as the electrons flow through right but the electrons are losing energy as they propagate through because of this Collision process now the theory of superconductors goes like this so here we have a superconductor right superconductor right so I need to draw kind of two different pictures I think to make it have it make sense I'm going to draw one on top and then I'm going to draw another one I think right down below again you have to use your imagination these are cartoons right this is not obviously not real but in the super connecting State the nuclei of the atoms that are in there they're not jiggling so much you cool everything down the violent motion of everything they are moving a little bit but as you get colder and colder and colder these things are in the lattice structure and a little more of an organized way and not jiggling around so much so they're more or less like this right and what happens is then if you send an electron in like right between them let's say then what's going to happen is because these things are not moving so much and any other electrons in the way are not having as much thermal energy then they're not there's not as many collisions right and so the electron has a little more of a clear path now this does not explain zero resistance but I'm just kind of setting it up for you here now I need to draw another situation as this electron makes its way through then what's going to happen is something like this let me draw it these little atoms are going to be a little closer together like this let me draw it I sent an electron in from the left here's another electron coming in the one that I already sent in is like maybe let's say right here you see when it gets to the center here Opposites Attract and this electron is sort of attracting the adjacent copper nuclei a little bit closer to itself and because of that it there's a little bit of a higher density of of if you look at this as a draw a boundary around it whereas before you had a neutral conductor because they're physically pulled a little bit closer to the electron and because there's not as much thermal motion to kind of swamp everything then from the outside it looks like there's kind of a net positive charge here because the these adjacent uh whatever material is superconducting a nuclei are getting a little bit closer to the charge that I just sent through so that means that this one when it goes and starts to enter in it's going to sort of see this negative charge but it's also going to see a overall positive charge over here and this negative charge will be weakly attracted to this net positive charge over here I'm going to say that one more time because that is really the core of the theory because the thermal agitation has stopped in some materials the electron can attract nearby positive nuclei remember the electrons are more or less Unbound I mean they're not really totally Unbound but they're very very Loosely bound so mostly you have the positive nuclei left behind and they're attracted making a overall positive region the incoming electron the next one I send in sees this and it's sort of attracted to it so as these two electrons travel through the lattice these two electrons begin to behave like a bound pair I'm going to say that again it's important the two electrons begin to behave as a unit because the first electron brings in some atoms a nuclei a little closer looking a little bit like a positive charge and even though two electrons repel there's an overall attraction because these this electron that's already gone through has attracted some positive neighbors and so the new one that comes in sees the overall net positive and it sort of gets attracted to it and it sort of follows the leader it follows the first electron and it behaves like a pair it's weakly bound to it because essentially it's following that net positive charge all the way through so the two electrons begin to behave as a pair it's called a Cooper pair this is called BCS Theory one of the names is cooper cooper pair he proposed this right and then you have to ask yourself well what would happen if two electrons which normally repel sort of start to behave like a single unit because two things in chemistry or physics are said to be bound together if they're in a lower energy State when they're together than if they're apart these electrons even though they normally repel each other start to behave like in a lower energy State when they're together like this but it's because of the first interaction sort of causing the second interaction to move along but these two behave like they are a pair so what you can do is you can sort of draw a like a little circle around these things and this is called a Cooper pair and then you might have another Cooper pair over here and another Cooper pair over here and another Cooper pair over here and the electrons stop behaving as single electrons that go through by themselves and they start behaving in pairs and it's just because you've cooled it down so much that the thermal motion of all the atoms normally going crazy have slowed down so that this effect can manifest without cooling it down it won't manifest because the thermal motions will break this Cooper pair the Cooper pair is very weak it only exists at low temps because of what we said and if you raise the temp a little the agitation of the atoms will just break the whole thing and then Quantum mechanically they won't behave as a pair anymore now remember what I said over here I said bosons an example of which is a photon has a spin of one electrons an example of a fermion is a spin of one-half but if you have two electrons that begin to behave like a single particle that's one then you have a spin of one half and another spin of one half what's that going to make a spin of one it starts to behave pairs of these things start to behave as uh photons or similar to photons right so what you have is you have an electron and you have an electron which will be bound together as a pair but this one has a spin of one-half and another electron which is bound to has a spin of one-half and all together that makes a spin of one so it doesn't really behave totally like an electron anymore it starts to behave like a boson it starts to behave sort of like photons behave remember photons can stack on top of each other photons can be in a low energy State you know the energy level diagram I drew I showed you in a laser they can be all in the same you can have a thousand trillion photons all on top of each other all in the same energy state so in this superconducting state because the Cooper pairs are behaving like one it's sort of like a phase transition because the individual fermions are starting to behave not like fermions anymore like bosons like photons behave kind of right and that means you don't think of them as two particles you think of them as one particle with a spin of one and that one particle is already in the lowest possible energy state it can't collide with anything and take any further energy out because Quantum mechanically it's already in the lowest possible energy state so they're all behaving like this and so you start to to behave to think of instead of individual electrons flowing through you start to think of them as some other particle with a spin of one which can all be now on top of each other and behave totally different Quantum mechanically and they're all flowing through as sort of a fluid flowing through the superconductor together and they're not interact acting with each other why just the same way photons don't really interact with each other they can be on top of each other they don't Collide they don't lose energy photons when you shine two flashlights they don't like Collide and lose energy because if they lose energy then the color of the light would be different the color of the light that we see is related to the wavelength which is related to the energy if photons Collide they would change colors we don't see that ever shine two lasers same color they pass right through each other electrons when they're Bound in this coup repair configuration can be right on top of each other Quantum mechanically and you might say that doesn't make any sense I don't like it well sorry superconductors are real they are real and none of you watching this just like me have ever really interacted with an electron on a one by one basis yes my body is made of trillions and trillions of electrons along with everything else but I have never really played with a single electron I don't really know how it works I don't really know or could ever examine how it fundamentally behaves when you have two of them bound together like this but the theory is that when they're bound together weakly like this they behave not as electrons they behave some more similar to photons so they don't bump in to the other nuclei because the nuclei is so small the almost the whole atom is empty space and they don't interact with other electrons because they're they're acting like spin one giant spin one conglomerate particles so you can almost think of the electrons not being individualized anymore they're almost like a new particle kind of like a dotted line around two of them knock a new particle that behaves kind of like a photon and so they don't lose any energy as they sail through the superconductor but if you warm the superconductor up then you change the thermal agitation this effect is broken coup repair is broken they suddenly behave like individual electrons it just so happens in the history of mankind we've only examined electrons individually we never knew they could behave in a superconducting pair because we never could cool anything down before 1911 cold enough to actually see the effect right now let me see if I said everything so I guess the only thing I want to point to is that instead of behaving like this like electrons and Anatomy you can't stack them on top of each other we we did in this case but they had different spins but if you try to put another one you can't because they'll have like the same location and the same Quantum uh thing now you might say oh the locations here is different than here but remember electrons are not balls they're wave functions the wave actually exists all the way around the atom we just draw it as balls because it's kind of our own our best analogy but they're not balls they're waves that extend around the atom here so these are kind of overlapping but they are different spins so they can do that but we cannot put a third one we have to do that by moving it farther away from the center whenever photons exist they're literally all at the same energy of Drew them separate but really they could all be on top of each other at the same energy at same place same time same energy same Quantum States the electrons can begin to behave like that and that's how they can go through without any disruption so we increase the 10 temperature we disrupt the whole thing if we push too much electric current through we exceed that critical current density remember I said there was a critical current you can't go beyond that somehow when you put too many electrons through you disrupt the Cooper pairs we don't know exactly how and then that turns the superconductivity off and the critical magnetic field does the same thing when we have too much of a magnetic field it generates eddy current somehow which is so high that it disrupts the Cooper Pairs and so on but I have to say BCS theory is a theory for type 1 superconductors it doesn't predict flux pinning it doesn't predict you know lots of other characteristics of type 2 superconductors so we don't have a good theory for type 2 superconductors but it's a good mental model to understand the basics I bet that the type 2 superconductivity Theory once it's finally made will have elements of BCS theory in it but it will be modified slightly that's just my guess all right I hope that you um I hope you've enjoyed this I have enjoyed teaching it to you when I was in college I used to make these things I used to play with them all the time it was super fun I certainly didn't understand as much as I do now about them but even now I don't know very much I mean honestly nobody really does but I hope that I've been able to summarize it in a way that was interesting for you the main idea is that when you have a superconductor the electrons that are going in do not behave like individual electrons they start to behave like a collective fluid and a collective fluid that behaves like a like a photon fluid and I don't like using the word Photon really I should be telling you that they behave more like a um they behave more like a bow like a Boson the electrons no longer behave as individual particles they behave as sort of super conglomerate particles which are kind of combinations of electrons that have different properties of their individual electrons it's kind of like quarks quarks you know there are three quarks in a proton and three quarks and a neutron quarks are different than the proton itself but when they're combined in a certain way they make this thing that we call a proton that has its own properties when the quarks different quarks with different characteristics are combined in a different way they make this thing called a neutron which is totally different well under certain conditions two of these electrons can pair up in such a way that as a unit they behave more like a sea of boson-like substances that can then flow without any resistance because they are not interacting with each other the electrons are not as a pair they're not interacting with their neighboring pairs anymore because they're all existing at the same energy level much like the way photons do all right that's it that's all I have I'd love for you to leave me a comment let me know was it too much was it too too little too detailed too boring please let me know follow me on to the next one learn anything at mathandscience.com

Channel: Math and Science

Views: 31,152

Keywords: superconductors, superconductivity, electrical resistance, diamagnetism, quantum mechanics, Cooper pairs, types of superconductors, energy, transportation, magnetic levitation, medical imaging, science, physics, engineering, electricity, electronics, magnetic fields, materials science, zero resistance, levitation, magnetism, quantum locking, quantum physics, meissner effect, quantum locking explained, cooper pair, quantum locking magnets, quantum locking ferrofluid, flux pinning levitation

Id: a3YCEo_QZXQ

Length: 81min 38sec (4898 seconds)

Published: Mon Mar 27 2023